Sloped roofs are typically covered with mineral-surfaced asphalt shingles, such as those described in ASTM D225 (“Standard Specification for Asphalt Shingles (Organic Felt) Surfaced with Mineral Granules”) or D3462 (“Standard Specification for Asphalt Shingles Made From Glass Felt and Surfaced with Mineral Granules”), to provide a water-shedding function while adding an aesthetically pleasing appearance to the roofs.
Such asphalt shingles are normally made from asphalt-impregnated fabrics. The shingles also typically include colored roofing granules to provide aesthetically pleasing coloration to the shingles as well as to impart strength and durability to the shingles. The roofing granules are typically also used in asphalt-based roofing articles to help protect the asphalt from the effects of incident ultraviolet radiation.
It is known in the art, however, that conventional roofing materials that include granules, such as the described asphaltic shingles, reflect little near-infrared (“NIR” radiation; λ=about 700 nm to about 2500 nm) radiation and consequently absorb substantial solar heat. This absorption increases with dark-colored shingles; while white-colored asphalt shingles may have a solar reflectance in the range of about 25% to about 35%, dark-colored asphalt shingles may have a solar reflectance of only about 5 to about 15%. This absorption of solar heat typically results in elevated temperatures in the environment surrounding the shingle.
This elevation in temperature in turn gives rise to so-called heat-island effects that increase the need for energy-consuming cooling systems to offset these heat-island effects. Heat-island effects caused by the solar heat absorption of a dark-colored roofing article can be significant—temperatures as high as 77° C. have been observed on the surface of black roofing shingles on a sunny day having an ambient temperature of only about 21° C.
Several efforts have been made to address the challenges created by the described heat-island effects. Some have proposed incorporation of application of white- or light-colored pigments or coatings directly to roofing articles to enhance the articles' reflectivity. Consumers of roofing materials, however, have shown a marked aesthetic preference for black or dark-colored roofing articles.
Others have proposed roofing articles or granules having metal flakes, such as aluminum flakes, adhered to their surfaces, to provide a radiation-reflective surface. Such approaches, however, impart reflectivity to the roofing article and are not aesthetically pleasing to consumers.
Some solutions to the heat-island problem include the use of infrared-reflective pigments, known in the art as “cool pigments,” on roofing articles to provide the roofing articles with desired levels of color and of NIR reflectance. These “cool pigments,” however, have certain drawbacks. First, dark-colored “cool pigments,” while aesthetically desirable, are comparatively high in cost, present a limited solar reflectance and are not available in jet-black hues. Others have suggested to use of roofing articles having NIR-transparent pigments disposed atop a NIR-reflective substrate. These solutions, however, are also high cost and possess limited solar reflectance when manufactured in the dark hues typically preferred by consumers.
Accordingly, there is an ongoing need in the field for improved roofing granules that possess desirable NIR reflectance characteristics while also providing an aesthetically pleasing appearance and allowing for the construction of dark-colored roofing articles. There is also a related need for processes capable of producing such improved roofing granules.